Electrostatic non-impact printers employ apparatus to form latent electrostatic images on an insulating surface. This electrostatic latent image attracts toner from a developer station to render a visible image. The toned image is then transferred to paper at a transfer station. Toner particles are generally either inductively or triboelectrically charged to a potential opposite to that of the latent image so that they will be attracted and adhere to the electrostatic image and not to the non-charged background image. In an alternate arrangement, the developer unit is biased to the potential of the charge image and then toner is adhered in non-charged areas.
Toner may be transferred to paper using either pressure transfer or electrostatic transfer. In pressure transfer, as described in Fotland and Carrish, U.S. Pat. No. 4,365,549 (Dec. 28, 1982) for example, the toner is pressed into the paper fibers under high pressure developed in a transfer nip. Typical peak pressures are several thousand pounds per square inch corresponding to nip roller loadings of a few hundred pounds per linear inch of nip. In a typical electrostatic toner transfer, the back, or non-imaged side of the paper is exposed to the field of a corona the polarity of which is opposite to that of the toner that adheres to the image. Transfer of the toner image takes place when the paper is in contact with the image surface.
In order to eliminate background toner in white areas, care must be taken to assure that all toner particles have a net charge. In high-speed operation, it is difficult to relatively uniformly charge all toner particles in the developer unit and thus some toner is deposited in background areas.
In many cases, it is desired to generate a high-density image using a relatively low image charge density. This feature provides for higher speed more efficient printer operation. The formation of high optical density images at low charge levels require a low specific toner charge level and thus can also result in the development of background toner.
Kegelman, U.S. Pat. No. 4,839,673 (Jun. 13, 1989) teaches the use of an ac corona treatment just prior to electrostatic image transfer. While the Kegelman patent teaches the use of an ac corona to minimize background, no mention is made of means to remove neutralized toner prior to electrostatic transfer.
The present invention has been made with the foregoing background in mind.
An object of the present invention is to provide a method for reducing toner scatter and toner background in electrostatically developed images. More specifically, the invention provides for the use of an alternating current discharge and physical removal of undesired toner from the surface of imaging members subsequent to latent image development and prior to toner image transfer.
Another object of the present invention is to provide a method and apparatus for improving image edge acuity or sharpness by the elimination of same or wrong sign toner developed at image edges as a consequence of fringing field toner attraction.
A further advantage is provided when employing colored toners and particularly those employing polyester resin as a binder. Such toners often exhibit non-uniform triboelectric charge characteristics and a change in color pigment generally results in a change in triboelectric surface characteristics.
In addition, the present invention greatly reduces background toner levels when printing at very high speed using very fine toner particles. Fine particles produce higher acuity images but, at high operating speed, lead to the formation of a dust cloud near the toning region. This dust cloud invariably results in some undesired toner deposition in uncharged areas of the toner receptive surface.
Furthermore, the present invention minimizes image degradation caused by toner and carrier aging effects that generally result in tribocharge degradation.
High speed electrostatic latent image development using triboelectric or corona charged toner suffers from background toner scatter due to the presence of small amounts of wrong sign toner as well as from image charge forces in background areas. The present invention neutralizes undesired toner charge that can then be easily removed from the surface of the latent image carrier.
Toner deposited on an insulating surface during printer operation may have two forms of countercharge. The desired form is countercharge that resides on the surface of the insulator as a latent image. Such latent image may be formed from the charging and optical discharge of a photoconductor or, alternately, from a remote image charge generator charging a dielectric surface. Charged toner is electrostatically attracted and bound to such surface charge to form the desired visible image. Undesired background toner, of either charge polarity, may be formed in areas having no surface charge. In this case, the deposited toner electrostatic mirror image resides in the conducting substrate adjacent the insulating layer.
It is the difference in countercharge location and, more specifically, the bound versus mobile aspect of the two types of countercharge that provides the opportunity to eliminate background toner. This is accomplished by providing a high charge density bipolar (positive and negative) charge near the toner surface. Charge bound to the surface of the insulator or photoconductor may not be discharged while the mobile charge at the insulator substrate may be neutralized by the charge at the toner surface. This essentially removes any background toner charge and thus permits the easy removal of background charge by, for example, suction means.